With a recent dramatic increase in network traffic, a wavelength division multiplexing (WDM) technology has frequently been utilized for transmitting large amounts of information. In the WDM network technology field, a network structure having a complicated topology such as an interconnection network or a mesh network may be realized by an optical add-drop multiplexer (OADM) device or an optical hub (i.e., a wavelength cross-connect (WXC) device), which may serve as a function to add or drop one or more wavelength channel branches or switch the wavelength routes to pass through optical signals without converting the optical signals into electric signals.
Further, it has been desired to transmit the optical signals without converting them into electric signals in the middle of the optical signal transmission in order to reduce cost in the optical WDM network. However, in a typical optical fiber for use in optical signal transmission, light beams travel at different speeds corresponding to their optical wavelengths. Hence, even if a transmitting terminal simultaneously transmits the light beams having different optical wavelengths, a receiving terminal receives the transmitted light beams at different times based on the different optical wavelengths. This phenomenon is called “wavelength dispersion”.
The optical signal is generally modulated before signal transmission. Specifically, the modulated optical signal has a certain bandwidth of a frequency; that is, the modulated optical signal is formed of light beams having different wavelengths within the certain bandwidth. Thus, the waveform of the received light beams may be deformed due to the wavelength dispersion. If the amount of deformation is too large, information carried by the light beams (i.e., the modulated optical signal) may not be received correctly. Accordingly, it is desirable to compensate the wavelength dispersion by allowing the light beams forming the deformed waveform to be passed through a dispersion compensation module (DCM) having a wavelength dispersion property opposite to that of an optical fiber residing in a signal transmission channel when the light is received by each of the nodes. With this technique, the light beams may be capable of being transmitted in a long distance without having deformation in the waveform of the light beams.
However, the amount of the wavelength dispersion appears to increase in proportion to the signal transmission distance of light beams. Further, different types of the optical fibers seem to have different properties. Accordingly, it may be necessary to prepare different dispersion compensators for various distances or different types of the optical fibers. Inexpensive dispersion compensators are generally formed of a passive component such as the optical fiber or the like. Thus, one type of the optical fiber may include a fixed property. Thus, it may be necessary to determine, in advance, where to appropriately arrange dispersion compensators having different properties. Therefore, dispersion compensation design is to determine where to appropriately arrange the dispersion compensators having different properties.
It is preferable that the compensation amount of the wavelength dispersion of the compensator match the amount of the wavelength dispersion necessary for the optical fiber subjected to compensation. However, in view of reduction in power consumption, a passive optical component such as a dispersion compensation optical fiber may frequently be utilized as a dispersion compensator. In such a case, it may be necessary to manufacture the dispersion compensator tailored for the optical fiber subject to compensation. However, the manufacturing of the dispersion compensator tailored for different types of the optical fibers may not be desirable in terms of cost. Thus, in general, a dispersion compensation menu includes discrete values of the compensation amount set at certain intervals so as to limit the selectability of the discrete values of the compensation amount. Accordingly, even if the value closest to the necessary compensation amount for the dispersion compensator is selected from the menu, the selected value includes an error of approximately half of the interval of the compensation amount. That is, a dispersion compensation result is likely to have both excessive and insufficient compensation parts.
FIG. 1 illustrates an example of the dispersion compensation result, and FIG. 2 illustrates an example of the compensation amount menu for the dispersion compensator, discrete values of which are set at 100 ps/nm intervals. In FIG. 1, N1 to N6 represent nodes, and lines connecting the adjacent nodes represent an optical fiber. Note that the lines between the adjacent nodes are hereinafter called spans. In FIG. 1, notations beneath the spans indicate a wavelength dispersion amount in the optical fiber (hereinafter called a “dispersion amount”), a compensating amount of the dispersion in the optical fiber (herein after called a “dispersion compensation amount”), and a (wavelength) dispersion amount in the optical fiber after the dispersion is compensated in the order from top to bottom. Further, in parentheses, “excessive compensation” is noted if the dispersion compensation amount is greater than the dispersion amount in the optical fiber, whereas “insufficient compensation” is noted if the dispersion amount in the optical fiber is greater than the dispersion compensation amount. For example, the dispersion amount in the optical fiber between the nodes N1 and N2 is 253 ps/nm, and the dispersion compensation amount closest to 253 ps/nm is 300 ps/nm in the dispersion compensation menu illustrated in FIG. 2. As a result, 47 ps/nm of the dispersion compensation amount indicates an excessive compensation amount.
Since the compensation amount menu is composed of the discrete values set at 100 ps/nm intervals, a compensation error of ±50 ps/nm is obtained. In,the recent typical network composed of optical fibers connected in a mesh configuration, the optical fibers having the insufficient dispersion compensation amounts and the optical fibers having the excessive compensation amounts may be randomly arranged.
Next, FIG. 3 illustrates an example of a network of nodes. As illustrated in FIG. 3, the network is composed of the nodes N1 to N7, and the notations beneath the spans connecting the adjacent nodes indicate the dispersion amount in the optical fiber (illustrated at the upper part) and the dispersion compensation amount (illustrated at the lower part).
FIG. 4 illustrates an example of an allowable range of accumulated dispersion. The allowable range may vary with the distance of the optical fiber, which is expressed by approximately equal spaced intervals and is therefore represented by the number of spans. FIG. 5 illustrates accumulated dispersion in the optical fiber at the endpoint of a path from an initial node to a terminal node. For example, if the initial node is the node N1 and the terminal node is the node N2, a value obtained by subtracting the dispersion compensation amount from the dispersion amount in the span of the optical fiber between the nodes N1 and N2, which is 5 ps/nm. In this case, since the number of spans between the nodes N1 and N2 is one, the allowable range of the accumulated dispersion corresponding to the number of spans being one is in a range of 70 ps/nm to −30 ps/nm as illustrated in FIG. 4. Accordingly, the 5 ps/nm obtained above falls within the allowable range, which indicates that the path from the node N1 to the node N2 may be capable of signal transmission or transmission capable. Likewise, if the initial node is the node N1 and the terminal node is the node N7, the number of spans between the adjacent nodes N1 and N2, N2 and N3, N3 and N4, N4 and N5, N5 and N6 and N6 and N7 constituting a path is six. Accordingly, the accumulated dispersion is 32 ps/nm, and the allowable range of the accumulated dispersion is in a range of 35 ps/nm to 5 ps/nm, which indicates that the path from the node N1 to the node N7 may be transmission capable.
However, if the initial node is the node N1 and the terminal node is the node N6, the accumulated dispersion is −3 ps/nm, and the allowable range of the accumulated dispersion is in a rage of 42 ps/nm to −2 ps/nm, which indicates that the accumulated dispersion of −3 ps/nm is not within the allowable range. As a result, the path from the node N1 to the node N6 may not be transmission capable.
This indicates that despite the fact that the path from the node N1 to the node N6 is incapable of signal transmission or transmission incapable, the path from the node N1 to the node N7, which is longer than the path from the node N1 to the node N6, is transmission capable.
FIG. 6 illustrates an example of a typical procedure for designing an arrangement of dispersion compensators. As illustrated in FIG. 6, the arrangement of dispersion compensators is generated in step S1, and whether all the optical paths are transmission capable is determined in step S2. If all the optical paths are transmission capable (“YES” in step S2), designing of the arrangement of the dispersion compensators is completed. However, if part of the optical paths are not transmission capable (“NO” in step S2), a regenerative repeater is added in the middle of the path that is transmission incapable (step S3).
FIG. 7 illustrates an example of another typical procedure for designing an arrangement of dispersion compensators. As illustrated in FIG. 7, an arrangement of dispersion compensators is generated in step S5, and whether all the optical paths are transmission capable is determined in step S6. If all the optical paths are transmission capable (“YES” in step S6), designing of the dispersion compensator arrangement is completed. However, if part of the optical paths are not transmission capable (“NO” in step S6), a process in step S5 is repeated after a constraint condition is added (step S7). Not that constraint condition is added for preventing the dispersion compensation amount of the transmission incapable optical path from being combined.
There is disclosed a wavelength dispersion compensation design technology for an arbitrary link including a plurality of spans extracted from the optical network and a plurality of nodes having the add/drop function (e.g., WO/2005/006604). In this technology, the wavelength dispersion compensation amount of the wavelength dispersion compensation device provided for each path is set so that all the residual dispersion range of the respective paths which have reached the respective nodes are within the allowable residual dispersion range set as the transmission enabled condition for all the paths of the link.
Further, there is disclosed an optical transmission network design technology to which a wavelength multiplexing transmission system is applied (e.g., Japanese Laid-open Patent Publication No. 2006-135788). In this technology, initial setup of the optical transmission network is inputted, and an arrangement pattern by which wavelength multiplexing variable dispersion compensators are arranged in the optical transmission network is obtained based on the setups. The sum of fixed dispersion values is obtained when a fixed dispersion compensator is replaced with the wavelength multiplexing variable dispersion compensator, and arrangement patterns are sorted in the order of priority based on an absolute value of the sum of the obtained fixed dispersion values.
Moreover, there is disclosed a network designing technology capable of obtaining the installation of a reproducing relay device optimal for a specific network while ensuring signal quality of a path inside a network (e.g., Japanese Laid-open Patent Publication No. 2006-42279). In this technology, a linear network is divided into a plurality of regenerating intervals each including nodes. In the regenerating intervals, regenerators are disposed in opposing ends, and devices such as an “n” optical amplifier, an OADM and the like are disposed for each of the nodes located in the regenerating intervals. In each of the regenerating intervals, a plurality of assumed paths obtained as a result of the arrangement are then extracted and the possibility of transmission is determined for each assumed path. The transmission possibility determination is displayed and reset by a user.
Patent Document 1: WO/2005/006604
Patent Document 2: Japanese Laid-open Patent Publication No. 2006-135788
Patent Document 3: Japanese Laid-open Patent Publication No. 2006-42279
Non-patent Document 1: “Optimization of Discrete Systems”, published May, 2000, Morikita Shuppan, Co., Ltd.
When an arrangement of devices within a network is designed, the network is divided into a linear configuration or a ring configuration. The linear or ring configuration of the network is, called a segment. In general, the arrangement of the dispersion compensators is designed based on the procedures illustrated in FIGS. 6 and 7 while the segments are fixed.
According to the procedure illustrated in FIG. 6, if not all the paths are transmission capable, the regenerative repeater is simply added in the middle of the path that is transmission incapable regardless of the length of the path in question in step S3. Thus, in the examples illustrated in FIGS. 3 to 5, the path from the node N1 to the node N5 is transmission capable whereas the path from the node N1 to the node N6 is transmission incapable. Thus, even if the path from the node N1 to the node N7 is transmission capable, the regenerative repeater is automatically added to the node N5 to simply divide the path into a path from the node N1 to the node N5 and a path from the node N5 to the node N6. Therefore, the cost may be increased due to an increase in the number of regenerative repeaters.
Further, in the procedure of the example illustrated in FIG. 7, a constraint condition for preventing the dispersion compensation amount of the optical path that is transmission incapable in step S7 from being combined is added. However, at this time, the optical path that is transmission incapable is not classified based on the length of the optical path (i.e., based on whether the optical path is long or short). As a result, the restriction condition is added to the optical path from the node N1 to the node N6 despite the fact that the path from the node N1 to the node N7 is transmission capable. Thus, it may not be easy to modify the arrangement of the dispersion compensators in which a long optical path is transmission capable and a short optical path is transmission incapable.
In such a design of the dispersion compensator arrangement, even if the longer optical path is transmission capable, the shorter optical path may not always be transmission capable. Thus, in the operations of the network, the transmission of the optical path is tested regardless of the length of the optical path every time the optical path is used for signal transmission despite the fact that the longer optical path in the same network is capable of transmission. Accordingly, such a task is unnecessarily burdensome and liable to cause errors in the network operations.
In the procedures illustrated in FIGS. 6 and 7, segments are not taken into account. Thus, there is a possibility that the shorter optical path may not be transmission capable despite the fact that the longer optical path is transmission capable in bridging portions between the segments of the network. In this case, even if the longer optical path is transmission capable, the shorter optical path also needs to be tested whether it is transmission capable every time it is used in the bridging portions between the segments of the network in the network operations. Accordingly, such an unnecessarily burdensome task, which is also liable to cause errors, may need to be performed in the network operations.